1. Field of the Invention
Pulsed nuclear induction spin echo techniques are generally well known, having been shown and described for example in U.S. Pat. No. 2,887,673 issued to E. L. Hahn and U.S. Pat. No. Re. 23,950 granted to F. Bloch, et al. These patents indicate that if a paramagnetic substance is located in a strong uniform magnetic field for a sufficient period of time so as to be in thermal equilibrium, the resultant magnetic moment present in the material is aligned in the direction of the field. If an RF field or "90.degree." pulse having a frequency equal to the charateristic or Larmor frequency of the substance is applied at right angles to the field, a torque is applied to the magnetic moment which causes it to be tipped or turned away from the direction of the field. The angle of tipping, that is the angle between the moment and the direction of the field, is proportional to the magnitude of the field and the time during which the pulse is applied. Upon release of the displacing force, the spinning nuclei urged toward realignment by the force of the magnetic field, now rotate or precess about the field in much the same manner as a tipped gyroscope. Another RF field or "180.degree." pulse is then applied to the sample, also at right angles to the field. After a time equal to the time interval between the occurrence of the 90.degree. and 180.degree. pulses, the sample develops spontaneously a magnetic field of its own which is also normal to the magnetic field and which rotates around the latter's direction. The strength of this rotating field builds up to a maximum and then decays and is picked up inductively as a "spin echo" signal.
Recent advances in the art have been directed to electron spin systems which operate in the microwave range. The difference between electron and nuclear spin systems is due to the difference in the mass of the electron as opposed to that of the proton. Accordingly, the precessional frequency of the spinning electron is much, much higher as compared to the proton. As such the 90.degree. and 180.degree. pulse is applied to an electron spin echo system in the form of microwave pulses having a carrier frequency in the range of the Larmor frequencies of the electrons present in the sample.
2. Description of the Prior Art
There are many examples of spin echo memory techniques and apparatus wherein additional or supplementary magnetic fields are introduced into the sample in addition to the unidirectional static magnetic field for either regulating or introducing additional field inhomogeneity for the purposes desired. Such an example is U.S. Pat. No. 2,714,714 issued to A. G. Anderson, et al. which teaches a selective method of eliminating noise and interference due to the presence of unwanted storage in a memory sequence, removing unwanted effects while preserving the desired echo signals in their purest and hence the most effective state and further provides for selective read out or destruction of "stimulated" and "mirror" spin echo signals.
Also U.S. Pat. No. 3,072,890 granted to W. V. Smith, et al. describes an electron spin echo storage system utilizing a dual mode low Q microwave cavity which contains a sample material having a suitable spin system and on which an external magnetic field is applied. The input pulses to the system are applied in the TE.sub.012 mode and the spin echo signal is coupled out in the TE.sub.021 mode. While the input and output modes are substantially isolated from each other, they both exhibit a relatively broad bandpass or low Q value and furthermore operate at the same resonant frequency.
While the above noted prior art apparatus operate satisfactorily, they do not envision the concept contemplated by the subject invention wherein an echo signal of a different frequency from that of the applied microwave pulses is obtained.